Was struggling to find this on a walk up Rhinog Fawr on a very windy and mizzly day, and then as soon as we sat down to have some bait there it was on a rock right in front of us! Located just over the stone wall edging the western side of Llyn Du, to the left of the stile. Additional comments/photos on Megalthic Portal show there are more on other rocks nearby but the descending clouds kept us from finding those, apart from one ‘possible’.